Induction cooktops are preferred over conventional cooktops because they are more efficient, have greater temperature control precision including very low temperature settings and cook food more evenly. In conventional cooktop systems, a heat source, such as an electric element or gas burner, is used to heat the cookware in contact with the heat source. This type of cooking system is inefficient because only the portion of the cookware in contact with the heat source is directly heated and the rest of the cooking utensil (e.g. pot or pan) is heated through conduction. This causes non-uniform heating throughout the cooking utensil and takes longer to reach a desired temperature needed for adequate cooking.
In contrast, induction cooking systems use electromagnetism to turn the cooking utensil into the heat source. A power inverter supplies an alternating current (A/C) having a predetermined frequency to the induction coil. The A/C causes a fluctuating magnetic field which induces a current on the bottom surface of the cooking utensil. The induced current on the bottom surface then induces even smaller currents (eddy currents) within the cooking utensil thereby producing heat throughout the cookware. In general, the frequency of the current remains uniform from the power inverter, to the coil, to the current induced on the bottom of the pan and finally to the current induced within the pan.
Integrated touch-key user interfaces may be used in an induction cooking system. For example, the integrated touch-key user interface may be a capacitive glass touch screen, an inductive touch screen, a resistive touch screen or an LCD touch screen. When a capacitive glass touch screen is used, a conductive input (e.g., human touch) must be used in order to be detected as a valid key touch. When a user touches the screen, electric charge is transferred from the user to the screen and the charge on the capacitive layer decreases. This decrease in capacitance on the layer may be how a controller detects a valid key touch.
However, when the user is touching the cooking utensil and pushes a key on the capacitive user interface, a noisy, distorted high frequency signal equal to that of the inverter drive for the cooking coil is detected and interpreted as an invalid key touch because the signal frequency is much faster than a human can press in a given interval. As shown in FIG. 1A, when a user touches a key on a user interface without the influence of a noisy, distorted high frequency signal, one key press is detected in one second and it is considered a valid key press. The appliance then may respond according to the valid key press. Alternatively, as shown in FIG. 1B, when a user touches a key on the user interface while touching the cooking utensil, the noisy, distorted high frequency signal is conducted and detected by the user interface. The user interface then detects a key press for each frequency peak of the signal. For clarity purposes FIG. 1B illustrates 10 key presses within a one second interval, however in reality the number of presses is directly related to the frequency of the inverter. More specifically, approximately 20-50 thousand presses may be detected within a one second interval because the frequency of the inverter is generally between 20-50 KHz. Because it is impossible for a human to activate a valid key press that quickly, the key press is rejected and the user interface continues to wait for an acceptable key press.
In view of these known concerns, it would be advantageous to provide an induction cooktop system with the capability to detect a valid key touch when noise is introduced into the system.